Apparatus for and methods of optical encoding having spiral shaped light modulator

Abstract

Disclosed are apparatus and methods for transducing mechanical movement into electrical signals indicative of the movement. The apparatus is an improved optical encoder of the type having a source of light, a code wheel for modulating the light according to movement of the object whose position is being measured and a photo detector for receiving the modulated light. The code wheel modulates the light from the light source into spiral-shaped beams of light separated by columns of spiral-shaped dark areas. Photo detectors are oriented to receive the modulated light.

What is claimed is: 
     
       1. An optical encoder for generating a signal reflective of the movement of an object along a predetermined route comprising: (a) a light source for emitting a light beam along a light path;   (b) moveable modulating means, positioned in said light path, for modulating said light beam in response to the movement of the object, said modulating means comprising an optical track positioned in said light path to provide spiral shaped light images separated by spiral shaped dark images; and   (c) an interdigitated photo detector array positioned to receive said modulated light and for generating said signal in response to light incident thereon, wherein said photodetector array is oriented with respect to said images such that the photo detector surface area on which said spiral shaped images are incident is substantially constant as said object moves along its route.   
     
     
       2. The encoder of claim 1 wherein: (a) said spiral shaped light images have leading edges and trailing edges; and   (b) said photodetector array comprises a plurality of adjacent photo detector areas oriented such that the portions of said leading and trailing edges which illuminate the photodetector array are substantially parallel to the interfaces between the photo detector areas.   
     
     
       3. The optical encoder of claim 1 wherein the object is a rotatable shaft and said modulating means comprises a code wheel coupled to the shaft for modulating said light beam in response to the rotation of the shaft, said code wheel having a circumferential track of spiral shaped windows separated by spiral shaped spokes positioned in said light path. 
     
     
       4. The encoder of claim 3 wherein said code wheel is located between said light source and said photo detector array. 
     
     
       5. The encoder of claim 4 wherein said windows comprise areas of said code wheel substantially transmissive to light from said light source and said spokes comprise areas of said code wheel substantially non-transmissive to light from said light source. 
     
     
       6. The encoder of claim 4 wherein said photo detector array is a rectangular photo detector array comprised a plurality of individual rectangular photo detector areas immediately adjacent to one another, said array being held in plane substantially parallel to the plane of said code wheel. 
     
     
       7. The encoder of claim 6 wherein the angle formed between a line parallel to the long side of said photo detector array and a radial line in the plane of said photo detector array which passes through about the center of the detector array is equal to about the value of θ determined as follows:   θ=sin.sup.-1 (N W/(2πR M))     where,   N is the number of counts per revolution of the code wheel,   W is the usable width of said photo detector array,   R is the optical radius of said photo detector array, and   M is the magnification of the projected light spiral image.   
     
     
       8. The encoder of claim 7 wherein the spiral images have leading edges and trailing edges shaped according to about the following equation in polar coordinates:   K=360(COS (θ)/(N W)),     origin of said polar coordinate system.   
     
     
       9. The encoder of claim 3 wherein said light source and said photo detector array are located on the same side of said code wheel. 
     
     
       10. The encoder of claim 9 wherein said spokes comprise reflective areas of said code wheel and said windows comprise substantially non-reflective areas of said code wheel. 
     
     
       11. The encoder of claim 10 wherein said photo detector array is a rectangular photo detector array comprised of a plurality of individual rectangular photo detector areas immediately adjacent to one another, said array being held in a plane substantially parallel to the plane of said code wheel. 
     
     
       12. The encoder of claim 11 wherein the angle formed between a line parallel to the long side of said photo detector array and a radial line in the plane of said photo detector array which passes through about the center of the detector array is equal to about the value of θ determined as follows:   θ=sin.sup.-1 (N W/(2πR M))     where,   N is the number of counts per revolution of the code wheel,   W is the usable width of said photo detector array,   R is the optical radius of said photo detector   M is the magnification of the projected light spiral image.   
     
     
       13. The encoder of claim 11 wherein the spiral images have leading edges and trailing edges shaped according to about the following equation in polar coordinates:   K=360(COS (θ)/(N W)),     where the axis of rotation of said code wheel passes through the origin of said polar coordinate system.   
     
     
       14. A method for generating a signal reflective of the movement of an object along a predetermined route comprising: (a) emitting a light beam along a light path;   (b) modulating said light beam in response to the movement of the object by positioning an optical track in said light path to provide spiral shaped light images separated by spiral shaped dark images; and   (c) positioning an interdigitated photo detector array to receive said modulated light whereby said interdigitated photo detector array is oriented with respect to said images so that the photo detector surface area on which said spiral shaped images are incident is substantially constant as said object moves along its route and whereby said signal is generated in response to light incident on said interdigitated photo detector array.   
     
     
       15. The method of claim 14 wherein: (a) said spiral shaped light images have leading and trailing edges; and   (b) said photo detector array comprises a plurality of adjacent photo detector areas oriented such that the portions of said leading and trailing edges which illuminate the photodetector array are substantially parallel to the interfaces between the photo detector areas.   
     
     
       16. The method of claim 15 wherein the object is a rotatable shaft and said modulating step comprises placing the optical track of a code wheel in said light path, said optical track comprising a circumferential track of spiral shaped windows separated by spiral shaped spokes. 
     
     
       17. The method of claim 16 wherein said code wheel is located between said light source and said photo detector array. 
     
     
       18. The method of claim 17 wherein said WindoWs comprise areas of said code wheel substantially transmissive to light from said light source and said spokes comprise areas of said code wheel substantially non-transmissive to light from said light source. 
     
     
       19. The method of claim 18 wherein said photo detector array is a rectangular photo detector array comprised a plurality of individual rectangular photo detector areas immediately adjacent to one another, said array being held in plane substantially parallel to the plane of said code wheel. 
     
     
       20. The method of claim 16 wherein said light source and said photo detector array are located on the same side of said code wheel. 
     
     
       21. The method of claim 20 wherein said spokes comprise reflective areas of said code wheel and said windows comprise substantially non-reflective areas of said code wheel. 
     
     
       22. The method of claim 21 wherein said photo detector array is a rectangular photo detector array comprised of a plurality of individual rectangular photo detector areas immediately adjacent to one another, said array being held in a plane substantially parallel to the plane of said code wheel.